Hydrogen generators are devices that generate hydrogen gas for use in fuel cells, combustion engines, and other devices, frequently through the evolution of hydrogen gas from chemical hydrides, borohydrides or boranes. Sodium borohydride (NaBH4) has emerged as a particularly desirable material for use in such devices, due to the molar equivalents of hydrogen it generates (see EQUATION 1 below), the relatively low mass of NaBH4 as compared to some competing materials, and the controllability of the hydrogen evolution reaction:NaBH4+2H2ONaBO2+4H2  (EQUATION 1)
However, despite the many advantages of NaBH4, its use in hydrogen generators is also beset by certain challenges. In particular, it is frequently found that a substantial amount of unreacted borohydride remains in spent generators. This unreacted material represents a significant decrease in the efficiency of the device, and an increase in the cost per unit of hydrogen gas produced by the generator. A similar problem is encountered in hydrogen generators based on other hydrogen-containing materials.
One apparent cause of this problem relates to the spent byproducts of the hydrogen generation reaction. In order to proceed, this reaction requires physical contact between the reactants, namely, the borohydride and water. Typically, the borohydride is provided in a granular or particulate form. Hence, for the reaction to proceed, water molecules must come into contact with the exposed surfaces of the borohydride granules. As the reaction proceeds, however, the sodium borate byproduct generated by the hydrolysis reaction forms a film of sodium borate (NaBO2) over the surfaces of the borohydride granules, thereby forming a physical barrier between the reactants and halting the reaction before it can proceed to completion. A similar phenomenon is observed with many other hydrogen-containing materials.
Some attempts have been made in the art to overcome this problem. For example, U.S. Pat. No. 6,811,764 (Jorgensen et al.) proposes a hydrogen generation system which utilizes grinding to expose unreacted borohydride for further reaction. However, this approach necessarily reduces the overall energy efficiency of a device that relies on the hydrogen generator as a fuel source, since some energy must be consumed in physically grinding the reactants. Moreover, such an approach is unsuitable for applications in which grinding of the reactants is impractical, due to size or noise limitations or to other considerations.
There is thus a need in the art for a method for improving the efficiency of hydrogen generators without the need for grinding the reactants, and for a system which utilizes this method. There is further a need in the art for a hydrogen generator of improved efficiency which is suitable for use in compact devices, and which does not generate noise. These and other needs are met by the devices and methodologies disclosed herein and hereinafter described.